Summary Tendons connect and transfer the force between the muscles and the bone. Their development, growth, and maturation must take place in coordination with that of their neighboring tissues. In adults, tendons are highly prone to injury and have slow and limited healing potential. By gaining a better understanding of the mechanisms regulating tendon biology we will be able to improve the design of regenerative based therapies for tendon injury and disease. Postnatal tendon cell growth and maturation is currently a poorly understood process. Many studies have focused on the changes occurring in the extracellular matrix as the collagen fibrils grow in size. However, few studies have examined the molecular changes taking place in the tendon cells at postnatal stages on a genome-wide level. In this collaborative proposal, we will apply unique and novel techniques to isolate tendon-specific cell populations and elucidate the gene networks and regulatory regions associated with a specific postnatal developmental transition. This transition was identified through our unpublished studies on cell turnover rates during postnatal and adult periods. We found that there is a specific stage at which the cells shift from high to low cycling rates that resemble the adult state of low turnover. At similar stages, we also observe distinct changes in gene expression levels. Interestingly, this transition stage correlates with alterations in regenerative potential reported in previous studies. Based on this, we will perform RNA-seq of postnatal stage tendon cells to identify transcripts that are differentially regulated between active and low cycling states. Candidates will be validated through gene expression analysis in mouse tendon tissues. Second, we will use ATAC-seq to identify genome-wide changes in regulatory control during high and low cell turnover periods. Regulatory regions will be tested using the zebrafish to identify enhancers with activity in tendon tissues. Together, these aims will reveal the gene expression and regulatory landscape changes that occur as the tendon cells transition from high to low cycling periods during tendon cell growth and maturation. These discoveries will enhance our understanding of tendon regulation on a molecular level and would provide new candidate pathways to test in the context of tendon cell growth, maturation and injury repair.